Technical Field
The present disclosure relates to the field of wind turbines. More particularly, but not exclusively, the present disclosure relates to the integration of two categories of wind turbines, Horizontal Axis Wind Turbines (HAWTs) and Vertical Axis Wind Turbines (VAWTs).
Discussion of Related Art
The invention is a refinement of the current method of harvesting wind energy using Horizontal Axis Wind Turbines (HAWTs) by integrating them with Vertical Axis Wind. Turbines (VAWTs) to facilitate a high density power resource. This heightens the wind speed limit that the HAWTs can withstand to produce electricity.
Horizontal axis wind turbines (HAWTs) are the primary source of grid connected wind power. To optimize the power drawn at varying wind speed, several control schemes have been devised. They include the implementation of combinations of rotor speed control algorithms for synchronous power generation using methods such as Maximum Power Point Tracking (MPPT) and Hill Climb Search (TICS). Primarily, they incorporate pitch mechanisms of the aerofoils to vary the angle of attack of wind. This controls the instantaneous Tip Speed Ratio (T.S.R) of the HAWT. Conventionally, at high wind speeds nearing the rated speed of the wind turbine, the HAWT's rpm may be stall controlled or pitch controlled to limit power generation, causing the wastage of a portion of potential wind power at a site.
The aerodynamics of Vertical Axis Wind Turbines (VAWTs) is more complicated. The scheme of arranging these turbines in arrays so that they may mutually aid each other in rotation is an area of topical research. Unlike their horizontal axis counterparts, VAWTs cannot access the stream of fast flowing wind as they are located at a lower elevation from the ground. Further, they have a smaller swept area than HAWTs and also suffer from turbulence at ground level. To save costs, intermediate gearboxes are avoided and instead low solidity ratio models which facilitate low torque, high rpm central shaft rotation to optimize the use of alternators, are used. These designs, however, do not increase the power capacity of VAWTs and they are generally fabricated to serve a purpose of 8-10 kW onsite power generation. They do little to challenge the crescendo of power consumption at the grid level. Few efforts have been made to fabricate VAWTs of megawatt scale.
Heavy and growing dependence on imported fossil fuels creates severe monetary problems apart from adding to energy insecurity. Many of their older low-capacity (<500 kW) wind turbines installed more than 10 to 12 years ago occupy some of the best wind sites. These turbines need to be replaced with more efficient, larger capacity machines to repower the wind farms. Maintenance costs tend to be higher for aging wind turbine generators. Breakdown of critical components severely affects machine availability and increases operational cost of these small capacity turbines.
Under normal operating conditions, a conventional HAWT rotor facilitates a prime mover rotating at an angular speed suitable for synchronous generation. When the main shaft's rpm dips below the reference rpm, auxiliary power is supplied to make it synchronous, or the pitch of its blades is varied to alter the Tip Speed Ratio (TSR), normally around 5 to 6 for a 3-bladed turbine. Under high wind speed conditions which are unsuitable for operation, the pitch is varied to change the angle of attack or in extreme cases, brakes may be applied to cease electricity generation until favorable conditions ensue. The energy supplied by the wind every second is proportional to swept area of the rotor and the cube of the prevailing wind speed. Therefore, by limiting the force of the wind by reducing the Tip Speed Ratio, the full potential of the wind is unutilized due to electrical limitations. Instead, if there were a mechanism by which the energy from this higher energy source could be siphoned to a lower energy source, as a sink, without wasting it on a dump load (often done in non-grid connected/local electrical storage systems), then this setup may be used like a regenerative system.
The invention introduces the concept of using a modified VAWT to optimize the power output of the HAWT with the objective of repowering the HAWT in a unique way. Electric power is simultaneously generated by the VAWT at a lower height location on the same support tower. Transfer of the excess torque exerted on the HAWT's main shaft, at high wind speeds, to the VAWT's central shaft is made possible by using a Continuously Variable Transmission (CVT) Contrary to a manual or a conventional automatic transmission, the operation of a CVT involves no torque interruption during change in angular speed. This is because has no explicit gears. It is capable of smooth acceleration with no jerking motion during the integration of the HAWT and VAWT. This progressive technology provides the option for operators to improve the profitability on their site.